Heads Up!

Heads up!

NASA to help satellite operators
watch
ups and downs of Leonids

Nov. 12, 1999: Meteorites
have become popular enough in recent years that they have worked
their ways into disaster films and
finally into commercials that parody the films. In the ads, what
appears to be a massive rock aimed at the planet turns out to
be a small cinder by the time it hits.

That's surprisingly close to the truth. Most of what we see
blazing through the upper atmosphere - including the Nov. 17-18
Leonids meteor shower
- poses no hazard to us.

Right: A woodcut depicts the impression people had of
the 1799 Leonids. The woodcut appears to show everything the
artist saw during the entire shower rather than the view at any
given instant. Links to 338x498-pixel,
85KB GIF.

Parents and Educators: Please visit
Thursday's Classroom
for lesson plans and activities related to this story.

"Leonids never make it to the ground," said Bill
Cooke, an engineer for CSC Corp. working in the Space Environments
Team at NASA's Marshall Space Flight Center. "They vaporize
at heights of 100 to 160 km," or 62 to 100 miles.

"If they never hit the ground, then what's the big deal?"
he continued.

The answer is that satellites in Earth orbit are at a slightly
greater risk of damage since they are above Earth's protective
atmosphere. The chances of getting pulverized by "the big
one," or even a small one, are minimal.

What poses a far greater hazard is the
potential for electrical short circuits when the impact from
a dust grain forms a plasma cloud around a satellite.

Left: An Air Force officer prepares for the
coming shower at the Leonids Environment Operations Center. Links
to 413x281-pixel, 30K JPG.
Credit: NASA/Marshall.

So to help satellite operators keep an eye on what's going
on during this year's Leonids, a special Leonids Environment
Operations Center will operate at NASA/Marshall for 90 hours
centered on the expected peak influx of Leonid meteors. Cooke
will be part of the team that also includes members of the U.S.
Air Force 50th Space Wing and Canadian meteor scientists. NASA,
the Department of Defense, the European Space Agency, and commercial
satellite companies like Iridium all support the project.

It won't be a meteor storm equivalent of the weather bureau,
Cooke cautioned, but an effort to provide "situational awareness"
to satellite operators.

The Leonids are the remains of comet Tempel-Tuttle which orbits
the sun every 33 years in a direction opposite that of the planets
and most other objects in the solar system. That means that objects
shed by the comet run into the Earth at a relative speed of 72
km/s (150,000 mph).

That means that even small grains from the Leonids can
form large, energetic plasma clouds when they strike a satellite.
The risk is greater than from most meteor showers, because most
comets and asteroids (and their debris streams) orbit the sun
in the same direction as the Earth and so have lower impact velocities.

The Leonids happen every year, but peak every 33 years after
the comet has shed a fresh load of litter for Earth to sail through.
Older material is moved sunward by tidal effects of Jupiter and
Uranus.

Records of the Leonids go back
as far as 1799, and possibly 1100. The meteor storm of 1833,
with more than 100,000 per hour, "marked the birth of meteor
science as we know it," Cooke said. The 1866 storm inspired
the song, "Stars Fell on Alabama." Storms skipped 1899
and 1932, then roared back in 1966 with an incredible storm that
peaked at 150,000 meteors an hour.

"These things can surprise you," Cooke said.

Partly because of the 1966 showing, a few experts in 1998
issued dire warnings that satellites would be sandblasted.

Nothing happened.

"This year there is concern but
much less panic," Cooke said. "There is a degree of
concern because this is the first meteor storm of the modern
space age." Only a few dozen satellites were operating in
1966. Today, several hundred are in orbit, and much of Earth's
communications network is tied through them.

Left: Target, Earth. Well, not quite. The Leonids are
more like a spiral of material left behind by comet Tempel-Tuttle,
and Earth just sails through part of the stream once a year.
For 1999, this would be the view for particles unfortunate enough
to have our planet in their way. It also shows why the Middle
East and Eastern Europe would be the best place to observe. Links
to 704x704-pixel, 106KB
JPG. Credit: NASA/Marshall.

Yet Cooke has reason to be calm about the potential for harm.
He went back to the historical record. During the Apollo lunar
landing program, NASA wanted to know the meteor hazard to the
Apollo manned spacecraft.

NASA/Marshall outfitted the upper stages of three Saturn I
rockets (the two-stage predecessors of the massive Saturn V)
as Pegasus satellites with immense wings that spread in orbit.
The wings were equipped with thin metal plates to act as discharging
capacitors when struck by a meteoroid to record impacts. Both
Pegasus 2 and 3 were still operating during the 1965 and 1966
Leonids.

In 1965, they recorded two impacts, and in 1966, one impact.

"But not a single impact occurred when the satellites
were exposed to the radiant," the direction of the Leonids,
Cooke said. They all happened when the spacecraft were on the
other side of the world and thus shielded by the Earth.

Right: An artist's concept of a Pegasus satellite
in orbit in the 1960s. Links to 640x480-pixel,
398KB JPG. Credit: NASA/Marshall.

"One impact when you're seeing 150,000 an hour gives
you an idea of the relative threat," Cooke said. "That
is, there's not much of a threat at all." Cooke estimates
that the chances of any one satellite getting hit range from
1 in 1 million to 1 in 10,000.

Still, as any statistician or
gambler will tell you, long odds don't translate into an absolute
zero chance. In August 1993, during the Perseids, an Olympus
communications satellite was lost to a meteor hit. The impact
formed a plasma cloud that caused electrical discharges in the
spacecraft and zapped its attitude control system. By the time
operators could stabilize it, they had depleted all of its attitude
control propellant and the satellite was a loss.

The only sure meteor protection for a
satellite is to keep it on the ground. But measures can be taken
to mitigate the risks for those in orbit. These include turning
off all but the most crucial electronics to reduce the risk of
short circuits in case of an impact, and pointing sensitive optics
and solar arrays away from the Leonids.

Left: The 1833 Leonids as they appeared of the coast
of Florida as witnessed by a government civil servant on his
way to New Orleans. Links to 289x440-pixel,
64KB GIF.

The degree of risk will vary with location. Those on the opposite
side of the Earth will be shielded from the Leonids (but can
still get hit from behind). Those in geostationary orbit will
be less exposed if their orbit has them on the dayside of Earth
since the peak flux is expected on the night side.

This year's Leonid watch will be a larger version of the 1998
storm watch that deployed teams equipped with radar and optical
instruments to Mongolia and northwest Australia on a line where
the Leonids were expected to peak.

Unfortunately, the estimate of peak rate was off by 16 hours
and the Canary Islands were the best place to observe (this year,
Eastern Europe and the Near East are expected to be the best
places). Cooke said the only fireball seen by the Mongolian team
was their tent burning after it caught fire when their hosts
used a wood stove to warm a truck's gasoline that had congealed
in the extreme cold. (No one was seriously injured.)

For 1999, a broader array of
gear will be deployed, including radar in (appropriately) Alert,
Canada. At 82 deg. N latitude, the source of the Leonids will
never set. Other observing stations have been set up in Israel,
the Canary Islands, Hawaii, Key West, and Kwajalein Atoll in
the Central Pacific. In addition, NASA's NC-135 Flying Infrared
Signature Technology Aircraft and an Air Force EC-18 Advanced
Range Instrumentation Aircraft will be airborne with optical
instruments to make stereo images of Leonid fireballs.

Data will be collected at the Leonids Environment Operations
Center at NASA/Marshall every 15 to 60 minutes - depending on
activity - and provided to the satellite user community over
the web. The web site will not be available to the public, Cooke
said, to keep it from being swamped by requests.

Right: A few civil servants, contractors and
military officers will have an entirely different view of the
Leonids than the many night watchers outside when they staff
the Leonids Environment Operations Center. Links to 469x352-pixel,
43KB JPG. Credit: NASA/Marshall.

That's OK because the center won't paint a pretty picture,
literally. Cooke said that most of the data will be tabular,
with the most important products being the flux numbers. A special
Leonid fluence calculator has been available since July that
lets operators input satellite position data and get an idea
of what's flying through their neighborhood. This fluence calculator
is used before the Leonids to estimate the risk to a particular
spacecraft.

There is no doubt that satellite
operators are glad the chance of damage is small, even if this
year produces a great show for sky watchers. Cooke noted that
not enough is known about the Leonids to predict exactly what
they will do, as witnessed by last year's disappointing show.

"We may be dealing with a very asymmetrical stream of
material," he said.

And we probably are dealing with the show of the decade, or
perhaps the next century, if it materializes as expected.

"After 2000, you won't see another one for about 100
years," Cooke said. "This year is your last chance
for a long time."

Stars Fell on Alabama
- Words
by Mitchell Parish and music
by Frank Perkins (you'll need something that can play mp3 sounds,
such as the free QuickTime
Player, to hear this snatch performed by Harry Connick, Jr.)
for the classic jazz tune.